1. Field of the Invention
This invention relates to a holder for fixing by suction thick-film substrates such as semiconductor substrates, alumina, and quartz.
2. Description of the Related Art
There are various types of conventional holders designed to abut a smooth surface of a held object and hold said piece, e.g., devices used in the manufacturing process of semiconductor IC circuits to carry and fix wafers to preset positions. In exposure systems playing a principal role in the lithography process, vacuum chuck apparatus are employed to fix thin-film wafers to inside a plane.
It is known that anodized aluminum alloys and alumina ceramics are the materials currently used for a contact part with the held object to be held by the holder of this type. It is also known that it is impossible for the holder the contact part of which is made of an aluminum alloy to maintain the accuracy of the suction surface thereof over a long period of time due to abrasion or the like, while the holder having an alumina ceramics-made contact part is also not free from difficulty in that when in use, the holding part becomes electrically charged attracting dust particles which transfer and contaminate wafers, diminishing suction accuracy of the held objects to be held.
As a result of micro-miniaturization in recent years, insofar as the semiconductor IC circuit is concerned, it is necessary for the wafer exposure surface to have a completely two-dimensionally flat and ultra smooth surface. Accordingly, it is required that the surface accuracy of the suction surface of the holder (vacuum chuck holder) holding the held object to be held (wafer) is in ultra smooth finish.
When the contact surface of the holder is shaped by machining, the aluminum alloy, despite being soft and easily machinable, readily deteriorates. Alumina ceramics tend to absorb dust particles due to the existence of pin holes and electric charging with a further disadvantage of a large coefficient of thermal expansion.
As explained in the aforementioned prior art techniques, the properties required of materials of the holders designed to hold the held object in contact with the smooth surface of holders are as follows:
(1) Excellent abrasion resistance, i.e., a high degree of hardness;
(2) No suction of dust particles due to the existence of pin holes and electric charging;
(3) Small coefficient of thermal expansion;
(4) Low specific gravity and high strength; and
(5) Excellent machinability. The foregoing properties are described hereinunder in detail.
The contact surface between the holder and the held object wears out and suffers damage through repetition of the holing operation. Especially when a thin piece such as a thin-film wafer is sucked to a vacuum chuck holder and held there, the surface of the wafer subjected to suction must be first corrected to the optimum position for the exposure system to produce light, that is, as perfect a two-dimensional plane as possible. Consequently, high abrasion resistance and high hardness are the desirable qualities of the contact surface between the holder and the held object to prevent deformation of the piece due to defects caused by abrasion of the contact surface as well as deformation of the piece due to presence of minute particles generated by abrasive wear resulting from contact therebetween. In case of wafers, deposits of minute particles upon the surface result in wire snapping and shorting in circuit printing.
Further, in addition to the aforementioned abrasive wear, dust being attracted to the electrically charged holder, i.e., infiltrating the pin holes on the ceramic surface, causes the same damage as mentioned above.
In case of the vacuum chuck holder of an exposure system, heat is accumulated due to the absorption of light at the time of exposure. As a result, a large coefficient of thermal expansion of the holder brings about a dimensional change of the holder proper, making it impossible to maintain the wafer in the state of being drawn by suction and fixed to the holder at the preset position. Hence, it is desirable for the coefficient of thermal expansion to be as small as possible to suppress dimensional errors in circuit printing.
When a large-sized held object is to be held, a load on the holding part of the holder becomes heavy, wherefore a material of high mechanical strength is needed for the holder, whereas this would result in increasing the thickness of the material so that an increase in strength runs counter to an endeavor to make the holder lightweight.
The most time- and labor-consuming step of the machining process of materials is the grinding step. Particularly, in the case of the vacuum chuck holder for holding wafers, the contact surface with the wafer must be ultra smooth because of correction required to attain flatness through vacuum suction. Therefore, grindability of the material of the holder becomes a critical factor.
Ceramic material have lately been developed as the materials meeting the above-mentioned prerequisite. However, since the existing ceramic materials which are generally very hard call for considerable amounts of machining as well as grinding energy to effect surface grinding to the level of ultra smoothness described above. Intensification of the grinding work tends to damage the polished surface, hence making it difficult to obtain a smooth and flat surface of high accuracy.